1. Technical Field
The present invention generally relates to satellite communication networks. In particular, the present invention pertains to a technique for multicasting packets through a satellite network constellation comprising plural satellites in plural orbital planes.
2. Discussion of the Related Art
There exists a plurality of multicasting protocol techniques. In general, these protocols belong to three major categories: source based multicast protocol, Core Based Tree (CBT) multicast protocol, and Forwarding Group Multicast Protocol (FGMP). In a source based multicast protocol, every node of the network has a multicast tree for transmitting multicast data to other nodes. In a CBT based multicast protocol, a single tree is created at the core of the network and the core is responsible for sending the multicast data. Finally, in the FGMP based protocol, a forwarding group is created that is responsible for forwarding the multicast data to other nodes in the network.
If all network nodes are stationary, there is no need to update an associated routing table. However, if the nodes of the network move, links between nodes may be broken, while new links may be created. Accordingly, modified link-state information must be forwarded to all nodes so that routing tables can be created and/or modified according to the new link-state information. The Dykstra Algorithm is typically employed to find the shortest or most cost effective path for routing the packets through the network.
Conventional techniques for multicasting through networks suffer from several disadvantages. For example, the above-mentioned multicast protocols are primarily directed toward Ad-Hoc networks. These types of networks do not include an infrastructure that is available to assist in routing data packets. Further, node mobility disrupts existing routing paths, where new link state information is flooded to enable the network to adapt to the changing environment. If the rate of the link state information updates are insufficient with respect to the speed of the node movement, there is significant performance degradation.
The primary performance issues pertaining to Ad-Hoc networks include scalability and reliability. Scalability refers to the ability of the protocol to function only on a smaller number of nodes and fail in response to significant increases in network size. Reliability refers to network performance, where the stability of completion rate and delay of packet transmissions may change due to node mobility, traffic, message size, etc. The scalability and reliability features require transmission overhead in order to build and maintain the network.
With respect to satellite constellation networks, these networks employ radio links over distances of tens of thousands of kilometers. Thus, the time required to propagate link state updates over the entire constellation network renders use of current multicast protocols impractical. Further, network performance degradation occurs with existing multicast protocols during periods of time that the link state information of each individual satellite is inconsistent with the rest of the constellation. Scalability is typically not an issue for a satellite constellation network since the total number of nodes in the network is relatively small compared to the number of nodes existing multicast protocols are designed to accommodate. Moreover, since the network overhead required to support scalability and mobility in existing multicast protocols is unnecessary for satellite networks, the protocols yield inefficient bandwidth utilization when applied to satellite networks.
The present invention essentially overcomes the aforementioned drawbacks. Initially, satellites move in predictable orbits based on known Keplarian parameters which simplifies routing calculations. The present invention takes advantage of the predictable satellite orbits for link assignments in order for satellite nodes to multicast packets to destinations throughout all the planes of a multi-plane satellite constellation. Link assignments refer to the manner in which a point-to-point link connection between specific pairs of satellites within a network may be determined based on the satellite orbits in a multi-plane satellite constellation. If there are broken links, packets are re-routed to bypass the broken links since each node typically has available multiple links to route packets. Unless all links of a satellite node are broken, a routing path is generally determined to transmit packets to the destination. This is accomplished without using bandwidth consuming link-state overhead traffic required by other multicast protocols. In addition, the present invention enhances packet completion rates in environments with link failures.